The invention relates generally to electronic manufacturing technology and, more particularly, to a solderability test apparatus for electronic components and its method of use for determining solderability.
A primary requirement of the manufacturing technology that is the underpinning of the electronics industry is the need to produce printed circuit boards having high quality and high reliability in a cost-effective manner with minimal rework. The problems encountered in meeting these requirements have been compounded by the rapid change of packaging technology from through-hole mounting to surface mounting of electronic components with very fine lead pitch that require xe2x80x9cpick and placexe2x80x9d mechanisms or other robotic means for accurate placement. Poor solderability of these components, or the surface onto which the components are mounted, such as a printed wiring board or ceramic substrate, lead to solder joint failure and ultimately to printed circuit board or ceramic substrate failure. A high priority in avoiding rework is crucial in this industry. Good solderability is a necessary factor in obtaining a wide enough process window to avoid rework. In the past, several different methods were developed to measure solderability of through-hole components.
A previous qualitative method for assessing solderability of component was developed for through-hole leaded components, and consists of a manual xe2x80x9cdip and lookxe2x80x9d test to determine if the component leads would or would not accept solder. This method involves dipping a component or printed wiring board into a pot of molten solder and examining the resulting soldered portion for the amount of wetted area. This solderability assessment is also based on the amount of nonwetted or dewetted area. As such, it tends to be subjective, dependent on the objectivity and interpretation of the operator, and therefore, not always reliable. Also very subjective and not always reliable is visual observation of a soldered connection for providing some assessment of solderability. These early qualitative approaches are no longer adequate for the electronics manufacturing industry.
Another method for assessing solderability is through the use of a wetting balance. This method is considered to be quantitative and is based on a time profile of the changes of forces exerted on a component or a mounting surface when dipped into a pot of molten solder. These forces include those resulting from buoyancy of the component or mounting surface due to displacement of the solder when dipped into the solder pot, and to changes in surface tension as the solder adheres to the component or mounting surface. The slope of the time profile curve and the time required to reach maximum force are important parameters for assessing solderability. The use of the wetting balance suffers from the inability to assess solderability of the newer fine-pitch surface mount components which may be leadless or have such small leads that the wetting balance is unable to make the measurements required to assess solderability.
Yet another method sometimes used for assessing solderability is sequential electrochemical reduction analysis, commonly referred to as SERA. Using this technique, a time profile of the voltage across a component lead or mounting surface is produced as an electric current is passed through the component lead or mounting surface and the oxides on the component or mounting surface are reduced by electrochemical means. Similar to the wetting balance method, the slope of the time profile curve and the time to reach a minimum voltage are important parameters for assessing solderability. This method suffers from drawbacks such as not all tin oxides reduce the same, organic solderability barriers affect the results, and different results are produced by different metals, while the noble metals do not work at all. This method does not work with small surfaces such as surface mount component leads.
Because of these limitations of existing solderability methods, there is a current 25 critical need in the electronics industry for a reliable and quantitative method and apparatus to assess the solderability of newer fine-pitch surface mount packages that may be xe2x80x9cleadlessxe2x80x9d or have such small leads that other methods are unable to make the measurements required for assessing solderability. There is also a need for an apparatus and method that provides a reliable solderability assessment for all tin oxides without being affected by organic solderability barriers, and that provides consistent results with all base metals. Furthermore, there is also a need for an apparatus and method that provides a reliable automated solderability assessment technology for these components.
The present invention is directed to an apparatus and method of use that satisfies these needs. The present invention provides for a reliable and quantitative method and apparatus to assess the solderability of newer fine-pitch surface mount components that may be leadless or have such small leads that other methods are unable to make the measurements required for assessing solderability. It also provides a method and apparatus that gives a reliable solderability assessment for all tin oxides without being affected by organic solderability barriers, and that gives consistent results with all base metals. The present invention also provides a reliable automated solderability assessment technology for these components.
The present invention provides a new methodology and apparatus for reliably and quantitatively assessing solderability of electronic components and mounting surfaces, addressing a critical need in the electronics industry. This invention is based on the use of the distinctive pattern in the infrared (IR) signature or radiation signal of a connection prior to and during wefting in a solder reflow process. The apparatus for assessing solderability comprises a mounting surface that is prepared by depositing solder paste on the surface. An electronic component is positioned onto the deposited solder paste. A means is provided for heating the mounting surface to a temperature that causes the solder paste to reflow. An IR camera or similar detector is positioned to view an image of the electronic component and the mounting surface, focusing on a component lead and the mounting surface. The heated IR radiation signal at a point in the image represents the radiation from that location on the component or mounting surface. When an ambient (unheated) IR radiation signal is subtracted from the heated IR radiation signal, the resulting IR radiation signal is dependent only on the temperature (above ambient) and the emissivity characteristics of the location of focus on the component lead and mounting surface. The parameter of interest when assessing solderability is the emissivity of the surface of the component connection. The emissivity of this connection varies when it is undergoing wetting by the molten solder, as monitored by the IR radiation signal. By recording selective pixels in the IR image that represent the component connection, a time profile of the IR radiation signals may be created that provides a basis for a solderability assessment.
A device having features of the present invention is an apparatus for assessing solderability, comprising a means for positioning a component having at least one lead on a solder paste deposit on a substrate, a means for heating the component, the component lead, the solder paste deposit, and the substrate, a means for measuring an IR radiation signal at the component lead and the substrate, and a means for determining acceptable solderability from the IR radiation signal. The means for heating may comprise a substrate heater positioned under the substrate. Power to the substrate heater may be adjusted in a controlled manner for achieving a predetermined temperature profile. The means for heating may comprise an IR radiation source. Power to the IR radiation source may be adjusted in a controlled manner for achieving a predetermined temperature profile. The means for measuring an IR radiation signal may comprise a device selected from the group consisting of an IR camera and an IR detector. The means for measuring an IR radiation signal may comprise capturing and storing in a computer memory gray level values at pixel locations in an IR image corresponding to locations on the lead and the substrate. The means for determining acceptable solderability from the IR radiation signal may comprise visually determining a peak magnitude value, visually determining a valley magnitude value, and computing a peak ratio. The means for determining acceptable solderability may further comprise comparing the peak ratio to predetermined peak ratio limit values. Acceptable solderability may be where the peak ratio is greater than a predetermined limit value of between 0.20 and 0.60. The means for determining acceptable solderability from the IR radiation signal may comprise visually determining a lead magnitude value at a temperature of less than 170xc2x0 C., visually determining a substrate magnitude value at the temperature of less than 170xc2x0 C., and computing a lead/substrate ratio. The means for determining acceptable solderability may further comprise comparing the lead/substrate ratio to predetermined lead/substrate ratio limit values. Acceptable solderability may be where the lead/substrate ratio is less than a predetermined limit value of between 0.40 and 0.60. The means for determining acceptable solderability from the IR radiation signal may comprise a computer image digitizer for converting the IR radiation signal into a digital code, a computer program for reading the converted IR radiation signal from the image digitizer and storing the digital code into computer memory, a computer program for detecting a sudden peak followed by a sudden valley in the converted IR radiation signal, a computer program for determining if a peak magnitude and a time from initiation of the sudden peak to the sudden peak are within predetermined limits, a computer program for determining if a valley magnitude value and a time from the sudden peak to the sudden valley are within predetermined limits, and a computer program for computing a peak ratio. The means for determining acceptable solderability may further comprise a computer program for comparing the peak ratio to predetermined peak ratio limit values. Acceptable solderability is where the peak ratio is greater than a predetermined limit value of between 0.20 and 0.60. The means for determining acceptable solderability from in the IR radiation signal may comprise a computer image digitizer for converting the IR radiation signal into a digital code, a computer program for reading the converted IR radiation signal from the image digitizer and stores the digital code into computer memory, a computer program for determining a lead magnitude value at a temperature of less than 170xc2x0 C., a computer program for determining a substrate magnitude value at the temperature of less than 170xc2x0 C., and a computer program for computing a lead/substrate ratio. The means for determining acceptable solderability may further comprise a computer program for comparing the lead/substrate ratio to predetermined lead/substrate ratio limit values. Acceptable solderability may be where the lead/substrate ratio is less than a predetermined limit value of between 0.40 and 0.60.
Another embodiment of the present invention is an apparatus for assessing solderability, comprising a component having at least one lead positioned on a solder paste deposit on a substrate, the substrate being positioned on a substrate heater for heating the substrate, the component, and the component lead, an IR camera positioned for viewing the substrate, the component, and the component lead, and a computer connected to the IR camera for determining a change in the IR radiation signal resulting from solder flux and liquid solder wetting the component lead. The apparatus may further comprise a variac connected to a power source and the substrate heater for manually adjusting a temperature of the substrate, the component, and the component lead. An operator may adjust the temperature of the substrate, the component, and the component lead in a controlled manner for achieving a predetermined temperature profile. The apparatus may further comprise a substrate heater power supply connected to a power source, the substrate heater, and the computer for automatically adjusting a temperature of the substrate, the component, and the component lead. The computer may adjust the temperature of the substrate, the component, and the component lead in a controlled manner for achieving a predetermined temperature profile. The apparatus may further comprise a data acquisition subsystem connected to thermocouples on the substrate, the output of a substrate heater power supply, and the computer for monitoring temperature and substrate heater power. The apparatus may further comprise an x-y table supporting the substrate for positioning the component, the component leads, and the substrate under the view of the IR camera. The apparatus may further comprise a video cassette recorder and a video monitor for recording and playing back time profiles of IR radiation signals from the IR camera. The computer may comprise an image digitizer for converting the IR radiation signal from the IR camera into a digital code, and a computer program comprising a means for reading the digital code, determining a peak magnitude value, determining a valley magnitude value, and computing a peak ratio value. The computer program may further comprise a means for comparing the computed peak ratio value with predetermined ratio values for determining solderability. The computer may comprises an image digitizer for converting the IR radiation signal from the IR camera into a digital code, and a computer program comprising a means for reading the digital code, determining a lead magnitude value, determining a substrate magnitude value, and computing a lead/substrate ratio value. The computer program may further comprise a means for comparing the computed lead/substrate ratio value with predetermined ratio values for determining solderability.
Another embodiment of the present invention is a method for assessing solderability, comprising positioning a component having at least one lead on a solder paste deposit on a substrate, heating the component, the component lead, the solder paste deposit, and the substrate, measuring an IR radiation signal at the component lead and the substrate, and determining acceptable solderability from the IR radiation signal. The heating step may comprise measuring a temperature at the substrate and adjusting the power to the substrate heater under computer control for achieving a predetermined temperature profile. The measuring step may comprise capturing and storing in a computer memory gray level values at pixel locations in an IR image corresponding to locations on the lead and the substrate. The determining step may comprise reading the IR radiation signals into a computer memory, detecting a sudden peak followed by a sudden valley in the IR radiation signal, determining if a peak magnitude value and a time from initiation of the sudden peak to the sudden peak are within predetermined peak limits, and determining if a valley magnitude value and a time from the sudden peak to the sudden valley are within predetermined valley limits. The determining step may further comprise computing a peak ratio value, and comparing the peak ratio value to predetermined ratio values. The determining step may further comprise computing a lead/substrate ratio value, and comparing the lead/substrate ratio value to predetermined ratio values.
Another embodiment of the present invention is a method for assessing solderability, comprising characterizing a solder paste reflow profile and solderability acceptability and unacceptability of a component type during an initial time interval of a reflow temperature profile up to a maximum temperature of 170xc2x0 C., determining predetermined upper and lower limits for an IR signal during the initial time interval of the temperature profile prior to reflow from a component lead with acceptable solderability and from a component lead with unacceptable solderability, and comparing an IR signal from a component lead of other components of the same component type during the initial time interval with the predetermined IR signal limits for determining solderability acceptability or unacceptability. The characterizing step may comprise reading the IR radiation signal from a component lead of the same component type into a computer memory, detecting a sudden peak followed by a sudden valley in the IR radiation signal, and determining acceptable solderability by determining if a peak magnitude value and a time from initiation of the sudden peak to the sudden peak are within predetermined peak limits, and if a valley magnitude value and a time from the sudden peak to the sudden valley are within predetermined valley limits.
Another embodiment of the present invention is an apparatus for assessing solderability, comprising a printed circuit board having at least one solder pad having a surface that is partially covered with a solder paste deposit, a means for heating the printed circuit board, the solder pad, and the solder paste deposit, a means for measuring an IR radiation signal at a part of the solder pad surface that is not covered with the solder paste deposit, and a means for determining a change in the IR radiation signal resulting from solder flux and liquid solder wetting the solder pad surface.